Level control systems appear in ever greater degrees of distribution in motor vehicles because these systems make possible a dip compensation of the vehicle chassis relative to the vehicle body, for example, during rapid acceleration operations or for a non-uniform loading state of the vehicle.
These systems include usually a control apparatus which is connected-via sensor lines to sensors for the direct or indirect determination of a distance of the vehicle chassis or vehicle frame relative to the vehicle axles and is connected to actuators via signal lines. With the aid of these actuators, the distance of the vehicle chassis relative to the wheel axis is adjustable in a targeted manner. The sensors are, as a rule, configured as displacement sensors; whereas, the actuators are hydraulically or pneumatically actuable piston-cylinder arrangements or air springs. Furthermore, a hydraulic or pneumatic pump belongs to such a system as do electric switching valves as well as a pressure medium store as may be required. A level control system of this kind is disclosed, for example, in United States patent publication 2002/0079626.
A so-called vehicle slant state is present for an alignment of the vehicle chassis to the particular vehicle axle with this alignment being transverse to the vehicle longitudinal axis. One such operating situation can, for example, be given in that the wheels of the right vehicle side stand on a curb stone while the opposite-lying wheels lie somewhat deeper on the roadway. U.S. Pat. No. 6,234,493 and European patent publication 0,752,329 describe the operation of level control systems in a situation wherein the vehicle slant state is, for example, only controlled out in the sense of a chassis alignment as horizontal as possible when the vehicle, after a vehicle loading operation, is driven down from the curbstone or when previously determined slant state limit values are exceeded.
Furthermore, German patent publication 100 22 834 discloses a method for preventing the turnover of an air-spring suspended vehicle on an inclined roadway. The operating situation described there is comparable to the above-mentioned curb stone situation. Accordingly, this publication relates to a vehicle having a level control system which is effective in a special manner when a vehicle having a high-lying center of gravity (that is, for example, a high-built commercial vehicle such as a bus) stands or travels with its longitudinal axis transverse to a slope.
In the method described in German patent publication 100 22 834, a turnover of the vehicle for a possible shift of weight within the tourist bus because of traveling guests is prevented in that all air springs of the vehicle are vented after the occurrence of specific control preconditions. According to this publication, in such a dangerous situation, however, it can also occur that only the higher-lying air springs on one side are vented and/or the lower-lying air springs on the other side are filled.
Finally, German patent publication 4,003,766 discloses a level control system for an air spring suspended vehicle wherein the level control operation is prevented in a selectable time span in such operating situations when the speed, with which a wheel axle of the vehicle moves relative to the vehicle frame, exceeds a pregiven threshold value. With this method, all those dynamic spring deflection operations with respect to the response of the level control system can be neglected which are generated by normal driving dynamic operations such as speed changes, travel in a curve, driving through potholes, et cetera.
In addition to the described slant state, there is especially in the operation of off-road vehicles an operating situation wherein the vehicle is not directed with its longitudinal axis precisely uphill or downhill, rather, where the longitudinal axis is aligned slanted to the hill. If such an off-road vehicle is equipped with a conventional level control system, the danger is present when exceeding a specific hill slope inclination angle and slant state angle that the vehicle assumes an unstable position with the level compensation unit and turns over.